This invention relates to belt operated media transport mechanisms, and particularly to computer printers, copiers, and the like.
Endless belts are common components in many different types of equipment. In such systems, a belt cycled over rollers generally needs to be steered or positioned on the rollers, i.e., lateral movement of the belt needs to be restrained, otherwise the belt will move partially or completely off the rollers during repeated cycling. Even when rollers are perfectly cylindrical, and a belt has a perfectly uniform circumference across its entire width, from edge to edge, belt centering is required. For imperfect components, centering is even more critical.
Conventional methods used to position belts include xe2x80x9cactive steeringxe2x80x9d and xe2x80x9cpassive steering.xe2x80x9d Active steering is a sensing device that feeds position information back to a steering roller. The steering roller then produces a change in the mechanism to produce a steering effect on the belt to correct for any deviation from the desired position. Devices to accomplish this tend to be complicated and/or costly, requiring an electronic control system to monitor feedback, to determine the required adjustment, and to make the required adjustment.
In addition, known active systems operate to divert the belt as it reaches a limit of excursion, so that the lateral position of the actively steered belt may vary unpredictably between established limits, and may undergo abrupt changes in lateral position as the limit is reached. This may be unsuitable for printers and other document generating equipment employing belts for carrying media or images for transfer to media, because abrupt lateral movements may impair image quality.
Passive steering is accomplished with a mechanism that biases the belt to a known and fixed position, or has guides that limit lateral excursion of the belt. The belt can be guided or limited by a flange on the roller or by another a stationary guide. This leads, however, to the eventual failure of the belt by wearing and cracking at the belt side edges that rub or press against the guides.
Non-metallic and other flexible or elastic belts can also be passively steered by xe2x80x9ccrowningxe2x80x9d one or more of the pulleys, which inherently centers the belt. However, this option will not work with metal and other inelastic or minimally flexible belts without deforming the belt material.
The present invention overcomes the limitations of the prior art by providing a media transport belt assembly. The assembly has a continuous belt encompassing two rollers supported on a frame. A tension mechanism biases the rollers apart to generate tension in the belt. One end of one roller moves in response to increased belt tension to rotate the first roller""s axis of rotation with respect to that of the second roller in a direction transverse to the plane of the belt. The roller end may be pivotally attached to a frame for movement in an arc, or may be otherwise constrained to allow motion in this direction.